The integument, which is composed of the skin, hair, and nails, provides a physical barrier to prevent the entrance of pathogens into the body. Many microorganisms live on the surface of human skin and make up the normal skin flora, also known as the skin microbiome. These microorganisms often participate in mutualism (relationships that benefit both the microorganisms and the human). By occupying the surface of the skin, these microorganisms prevent other, more harmful organisms from occupying that same space; in return, the microorganisms get a stable environment with access to nutrients. However, some of the normal flora can become pathogenic if they penetrate the integument.
The skin itself is also a nonspecific defense mechanism that protects against pathogenic invasion. Sebaceous glands in the skin secrete oil onto the surface of the skin to keep its pH relatively acidic (a range of approximately 4–6), which decreases bacterial growth. These secretions also help keep the skin moist. Sweat is secreted from other glands and helps cool the skin by evaporative cooling. Sweat also contains enzymes that help destroy bacterial cell walls as well as pheromones used in chemical communication among humans.
The skin is divided into two different layers, the dermis and the epidermis, which are connected together by the basement membrane. The dermis contains the blood supply to the skin and most of the specialized cells, whereas the epidermis contains mainly keratinocytes, which differentiate into corneocytes: protective, waterproof cells that do not undergo further replication and are routinely sloughed off and replaced.
Parts of the skin are covered with hair, which aids the skin with the above functions. Hair serves to direct sweat and waste away from the skin, helping with evaporative cooling when the body is hot. Conversely, hair also traps heat, preventing the body from becoming too cool. Hair also serves as a sensory organ, allowing the detection of nearby motion.
Other components of the integument help prevent infection as well. Mucous secretions in the nose and aqueous secretions in the eyes help prevent foreign organisms from entering the body and resist infection. Similarly, enzymes in the mouth and throat break down many entering microorganisms and decrease their pathogenicity. Finally, nails protect the tips of the fingers and toes from physical injury and can be used as tools.